1. Technical Field
The present invention relates to a surface acoustic wave resonator, a surface acoustic wave oscillator in which the resonator is mounted, and an electronic apparatus, and more particularly to a type of surface acoustic wave resonator where grooves are provided in a substrate surface, and a surface acoustic wave oscillator in which the resonator is mounted.
2. Related Art
In a surface acoustic wave (SAW) device (for example, a SAW resonator), the effect of a SAW stop band, piezoelectric substrate (for example, quartz crystal substrate) cut angle, IDT (interdigital transducer) formation shape, and the like, on changes in frequency-temperature characteristics is considerable.
For example, a configuration exciting each of a SAW stop band upper end mode and lower end mode, the distribution of standing waves in each of the stop band upper end mode and lower end mode, and the like, are disclosed in JP-A-11-214958.
In addition, points for which the SAW stop band upper end mode has better frequency-temperature characteristics than the stop band lower end mode are described in JP-A-2006-148622, JP-A-2007-208871, JP-A-2007-267033 and JP-A-2002-100959. Then, it is described in JP-A-2006-148622 and JP-A-2007-208871 that, in order to obtain favorable frequency-temperature characteristics in a SAW device utilizing a Rayleigh wave, as well as adjusting the cut angle of the quartz crystal substrate, the electrode standardizing film thickness (H/λ) is increased to around 0.1.
Further, it is described in JP-A-2007-267033 that, as well as adjusting the cut angle of the quartz crystal substrate in a SAW device utilizing a Rayleigh wave, the electrode standardizing film thickness (H/λ) is increased by around 0.045 or more.
In addition, it is described in JP-A-2002-100959 that, by using a rotated Y-cut, X-propagating quartz crystal substrate, and utilizing the stop band upper end resonance, the frequency-temperature characteristics improve more than in the case of using the stop band lower end resonance.
It is described in JP-A-57-5418 and “Manufacturing Conditions and Characteristics of Grooved SAW Resonators” (Institute of Electronics and Communication Engineers of Japan Technical Research Report MW82-59 (1982)), that grooves are provided between the electrode fingers configuring the IDT, and between the conductor strips configuring the reflectors, in a SAW device using an ST cut quartz crystal substrate. Also, it is described in “Manufacturing Conditions and Characteristics of Grooved SAW Resonators” (Institute of Electronics and Communication Engineers of Japan Technical Research Report MW82-59 (1982)), that a peak temperature in the frequency-temperature characteristics having a quadratic curve shape changes depending on the depth of the grooves, and a second-order temperature coefficient is approximately −3.4×10−8/° C.2.
In Japanese Patent No. 3,851,336, as well as describing a configuration for making a curve indicating the frequency-temperature characteristics a cubic curve in a SAW device using an LST cut quartz crystal substrate, it is described that, in a SAW device using a Rayleigh wave, it has not been possible to find a cut angle substrate having the kind of temperature characteristics indicated by a cubic curve.
As described above, there is a wide range of elements for improving the frequency-temperature characteristics, and it is thought that, particularly with a SAW device using a Rayleigh wave, increasing the film thickness of the electrodes configuring the IDT is one factor contributing to the frequency-temperature characteristics. However, the applicant has found experimentally that on increasing the film thickness of the electrodes, environmental resistance characteristics, such as temporal change characteristics and temperature and shock resistance characteristics, are deteriorated. Further, when having the improvement of frequency-temperature characteristics as a principal object, it is necessary to increase the electrode film thickness, as described above, and an accompanying deterioration of temporal change characteristics, temperature and shock resistance characteristics, and the like, is unavoidable. Since this also applies to the Q value, it is difficult to realize a higher Q without increasing the electrode film thickness.
In order to solve the problem, Pamphlet of International Publication WO2010/098139 discloses a configuration in which grooves are formed on a quartz crystal substrate in a direction perpendicular to the propagation direction of the surface acoustic wave, and electrodes are formed on convex portions formed by the grooves. With this, environmental resistance characteristics such as temporal change characteristics and temperature and shock resistance characteristics are improved, thereby realizing a high Q value. In addition, JP-A-61-220513 or JP-A-61-220514 discloses a configuration in which, in order to realize a high Q value, grooves are formed between stripe-shaped metal films constituting reflectors which are disposed between IDT electrodes or at both sides of the IDT electrode.
Further, in Pamphlet of International Publication WO2010/098139, a groove depth, a film thickness of an electrode formed on the groove, and a line occupation rate of the groove are systematically investigated. In addition, in a case where the surface acoustic wave resonator is excited in a stop band upper end mode, the condition is ascertained that an absolute value of the second-order temperature coefficient of the surface acoustic wave is 0.01 ppm/° C.2 or less by adjusting the line occupation rate with respect to a given groove depth and electrode film thickness. Thereby, since the frequency-temperature characteristics of the surface acoustic wave show a cubic curve, it is expected that a frequency deviation can be suppressed in a temperature range around the inflection point.
In addition, JP-A-2009-225420 discloses a configuration in which a frequency deviation in an operating temperature range of the surface acoustic wave resonator is reduced when the line width of the electrode finger constituting the IDT electrode, that is, a line occupation rate fluctuates.
However, there is a strong demand for reduction in loss in the surface acoustic wave resonator in the surface acoustic wave resonator disclosed in Pamphlet of International Publication WO2010/098139, JP-A-61-220513, JP-A-61-220514, and JP-A-2009-225420 as well, but there is no detailed disclosure thereof at present.